The field of the invention generally relates to test equipment pin drivers that produce driver pulses to test a device under test, and more particularly relates to apparatus and method for enhancing driver pulse parameters including pulse reflections, switching speed, and ringing.
As is well known, the role of a pin driver in typical automatic test equipment (ATE) systems is to send driver pulses having the correct voltages and timing to a device under test (DUT). A prototype pin driver was built to include a pulse forming section, a buffer, and an amplifier connected in series. The pulse forming section received a data input and, in response thereto, provided output pulses timed in accordance with the data input. The amplifier required a significant amount of input bias current, so an intermediate buffer was used to make the operation of the pulse forming section independent of the test device load. The amplifier included npn and pnp emitter follower transistors with the emitters being coupled together through two emitter resistors connected in series. The output of the amplifier was coupled from a node between the two emitter resistors though an output series resistor.
The use of an output series resistor is the generally accepted method of terminating ATE systems, and is referred to as series termination. That is, a pin driver is generally coupled to the pin of the device under test using a transmission line such as coaxial cable or microstrip line, and the line is terminated with a series resistor at the sending end. If the sum of series resistor value and the pin driver output impedance exactly equals the transmission line impedance, no reflections are generated. For this ideal condition, the pulse at the device under test would be a faithful replica of the pin driver output except for a delay time of approximately 1.6 nanoseconds per foot for a 50 ohm transmission line. Ideally, the output amplifier impedance would be zero, in which case the series resistor would be 50 ohms for a 50 ohm transmission line. In practice, however, output amplifier impedances are 7 ohms or larger, so output series resistors may generally be 43 ohms or less.
One problem with the above described prototype pin driver was that the pin driver output impedance increased during the leading and trailing edges of output driver pulses. That is, the output amplifier impedance did not stay constant at 7 ohms, or any other value. As a result, the sum of the series resistor value and the pin driver output impedance did not stay exactly equal to the 50 ohm transmission line, so the pulse waveform was distorted by reflections.
In the above described prototype pin driver, the bases of the respective amplifier emitter follower transistors were driven by respective npn and pnp emitter follower transistors in the buffer, and each buffer transistor emitter had a current source connected thereto. Turn on base currents for the amplifier transistors were respectively provided by the two buffer current sources. Turn off base currents for the amplifier transistors were respectively provided by the buffer transistors. A problem with this arrangement was that the pnp buffer transistor had better drive capability for negative going transitions than for positive, and the npn transistor had better drive capability for positive going transitions than for negative. Consequently, the amplifier transistors were not receiving equal input drive signals for both positive and negative going transitions. That is, one amplifier transistor was driven harder for positive going transitions and the other was driven harder for negative going transitions. The net result was that the switching speed of the output amplifier was not as fast as it could be.
Another problem was that in a typical packaged integrated circuit, the supply voltages V.sub.CC and V.sub.EE are brought to the integrated circuit using bonding wires. The series inductance presented by these wires could be as high as 4 nanohenries. For high current circuits such as pin drivers, inductances on this order could cause the driver pulses to have higher overshoot and slower rise times.